In a switched connection network, examples of which include the public switched telephone network and virtual connection orientated digital communications such as asynchronous transfer mode (ATM) networks, an end-to-end call comprises one or more switched (i.e., on-demand) bearer channels which collectively compose a bearer channel path across a network. In an ATM network for instance, examples of such a switched call include a switched virtual connection (SVC) or a soft permanent virtual connection (SPVC). Such calls are (or can be) dynamically established and cleared in substantially real time by network elements, such as data transmission switches, in accordance with standard signalling protocols. An example of one such network element which incorporates signalling software is the model 36170 MainStreet Xpress(TM) ATM network switch commercially available from Newbridge Networks Corporation of Kanata, Ontario.
The signalling between network elements is carried over a signalling network comprising call control and processing infrastructure disposed on each network element, and means for interfacing or communicating between similar infrastructure disposed on counterpart network elements. The interface means can comprise a separate overlay network, such as leased lines, as may be found in a Frame Relay SVC service. More typically, however, the interface means comprises a permanent virtual connection (PVC) which has been dedicated for the transfer of signalling information or call control data between interconnected network elements. For example, one popular ATM standard has dedicated VPI/VCI=0/5 for this purpose. Signalling virtual circuits can be carried over the same internode link facilities as the bearer channels, or on separate links.
In order to initiate an end-to-end call, the calling device typically transmits a "call setup" message to the network indicating the destination address and desired connection and quality of service parameters. For SVC service, the calling and called devices are typically customer premise equipment (CPE). For SPVC service, the calling and called devices are typically ingress and egress network elements, as described in greater detail below.
The call set up message can be propagated through the network to a called device (destination address) using conventional routing techniques, such as hop-by-hop or source routing. In hop-by-hop routing, each network element which receives the call setup message typically consults a routing table in order to determine the next hop or output bearer channel towards the destination. In source-routing, the source or ingress network element maintains a database of the topology of the network and specifies the output ports and virtual path (VP) trunk that each network element should use to route the call.
Each network element which receives the call setup message establishes a bearer channel cross-connect which links an input bearer channel to an output bearer channel. Ultimately, the call setup message is relayed by the signalling network to the called device, and the called device is thus informed as to the identity of the bearer channel it should use for transmitting information in respect of the call. Once the call setup message is received at the destination device, a "connect" message is sent back over the signalling network to the calling device. Typically, the calling device will then transmit a "connect acknowledgement" message back to the destination device over the signalling network in order to complete a double handshaking protocol. At this point, the call is deemed to have been established and the calling device, as well as the called device in the event of a bidirectional connection, may transmit user data over the recently established bearer channel path.
The signalling network is also used to clear or terminate a call and its associated bearer channel path in a manner similar to that used to establish the call.
In addition to dynamically establishing and clearing a call, the signalling network is also used to transmit various types of status messages (e.g. link state messages) relating to the call and the bearer channel path thereof. These status messages are associated with various sensing mechanisms employed by the signalling standards for determining whether a peer entity (e.g., a signalling module on a CPE or network node) or link therebetween is alive and properly functioning. Such mechanisms typically include heartbeat processes relating to various layers of a signalling protocol, such as described in greater detail below.
The signalling network, or more particularly a portion thereof, may fail for various reasons, including a software defect or equipment failure in the call control infrastructure. When a failure is sensed as described above by other network elements adjacent to the failed portion of the signalling network, the signalling standards typically specify that all calls affected by the failure should be released, thus causing all of the bearer channel cross-connects relating to those calls to be released. If a call control entity, for example, a call processor supporting switched virtual circuit (SVC) services on a first network element fails, all of the signalling interfaces with other network elements managed by the card will be lost. Adjacent network elements will thus presume that the bearer channels associated with the failed signalling interfaces are no longer operable. This causes the adjacent network elements to signal this fact across the network and release all cross-connects to the bearer channels composing the call. Ultimately, the failure in the signalling network will be signalled back to the calling and called devices, which will terminate their session.
The release of bearer channel cross-connects is very disruptive to calls if there has in fact been no failure in the bearer channels or their cross-connects (i.e., the bearer channel path is up and functioning) and the failure in the signalling network can be corrected within a short period of time. For example, in network elements featuring "1+1 warm redundancy" or "N+1 warm redundancy", backup or stand-by call control infrastructure, e.g. a stand-by SVC service card, could be switched into service in place of the failed card. Unfortunately, as the information base on the failed card is lost, the prior art does not enable the backup card to carry on in midstream the numerous signalling functions handled by the failed call control infrastructure.
One approach to this problem is to employ "1+1 Hot Redundancy" where each component of the call control infrastructure is actively duplicated in which case bearer channel cross-connects need not be released since the redundant component maintains a parallel up-to-date information base. Such systems are, however, not always available or desired due to the extra costs involved.